It is desirable to provide a broadband transducer that is capable of operating at a wide range of frequencies without a loss in sensitivity. As a result of the increased bandwidth provided by a broadband transducer, the resolution along the range axis may improve, resulting in better image quality. One possible application for a broadband transducer is contrast harmonic imaging. In contrast harmonic imaging, the heart and muscle tissue are clearly visible at a fundamental frequency, however, at the second harmonic, the contrast agent itself can be viewed.
Because contrast harmonic imaging requires that the transducer be capable of operating at a broad range of frequencies (i.e. at both the fundamental and second harmonic), existing transducers typically cannot function at such a broad range. For example, a transducer having a center frequency of 5 Megahertz and having a 60% ratio of bandwidth to center frequency has a bandwidth of 3.5 Megahertz to 6.50 Megahertz. If the fundamental harmonic is 3.5 Megahertz, then the second harmonic is 7.0 Megahertz. Thus, a transducer having a center frequency of 5 Megahertz would not be able to adequately operate at both the fundamental and second harmonic.
In addition to having a transducer which is capable of operating at a broad range of frequencies, two-dimensional transducer arrays are also desirable to increase the resolution of the images produced and allow three-dimensional imaging. An example of a two-dimensional transducer array is illustrated in U.S. Pat. No. 3,833,825 to Haan issued Sep. 3, 1974. Two-dimensional arrays allow for increased control of the excitation of ultrasound beams along the elevation axis which is absent from conventional single-dimensional arrays which only allow for control of the excitation of ultrasound beams along the azimuth axis.
However, two-dimensional arrays are difficult to fabricate because they typically require that each element be cut into several segments along the elevation axis. In addition, separate leads for exciting each of the respective segments must be provided. As an example, Haan describes a two-dimensional transducer array that has 64 elements, 8 segments in both the elevation and azimuth directions (i.e., 8.times.8 array). Of course 64 leads must also be provided to excite each of the 64 segments. This results in an 8-fold increase in the number of leads needed compared to a conventional single-dimensional array. If more segments are provided, more interconnecting leads must also be provided. In addition, such a two-dimensional array requires rather complicated software in order to excite each of the several segments at appropriate times during the ultrasound scan.
Also, because of the numerous diced segments in N.times.N arrays such as that described in Haan there results a very high impedance which makes it very difficult to electrically match the transducer to the ultrasound system which typically has a low impedance.
Conventional one-dimensional arrays have been used to perform two-dimensional scanning. In order to scan two-dimensionally, the array must include a positioner or provide for mechanical registration of the transducer's location in order to identify the location of each scan. Real-time three-dimensional imaging is therefore not possible with conventional one-dimensional transducers since all of the scan information is processed after it has been acquired. In addition, using a conventional one-dimensional transducer to perform two-dimensional scanning requires that the transducer be physically moved or tilted in position as each frame is acquired. Typically one frame can be acquired in about 33 milliseconds. It takes much longer than that for a human operator to physically move or tilt the transducer from scan to scan. Thus, the possibility of performing real or quasi-real time three-dimensional imaging is comprised. Also, the accuracy and reliability of positioners and mechanical registration can compromise the ability to obtain three-dimensional imaging.
It is therefore desirable to provide a two-dimensional transducer array that has the performance of an N.times.N array without the complexity of requiring N.times.N number of hardware channels or cables.
It is also desirable to provide a two-dimensional transducer array that is simple to manufacture and operate.
It is also desirable to provide a two-dimensional transducer array that can generate real-time three-dimensional images.
It is also desirable to provide a two-dimensional transducer that has a low impedance and therefore can be easily and inexpensively electrically matched to an ultrasound system.